Hard disk drive storage devices enable computer systems to quickly store data in a non-volatile manner and to retrieve the stored data when needed. The ongoing industry trend is toward computer systems with increased performance, which mandates disk drives with increased data access speeds.
A hard disk drive typically comprises pivotally mounted disks having a magnetic recording layer disposed thereon and a magnetic head elements for affecting and sensing the magnetization states of the recording layer. The recording layer comprises concentric circular tracks with data written to or read from each track by positioning a transducer head over the disk at a corresponding radius while the disk is rotated at a fixed angular speed. The time required to reposition the head is known as the “seek time” of the drive, with a shorter seek time generally translating into shorter data access time.
To position the head over a desired track, a head stack assembly (HSA) is used that includes a pivotally mounted actuator arm that supports the head, a voice coil motor (VCM) for exerting a torque onto the actuator arm, and a servo-controller for controlling the VCM and the movement of the actuator arm by directing a control current to flow through the coil which generates a torque that moves the actuator arm. The direction of the torque is dictated by the direction of control current flow, thus enabling the servo-controller to reposition the head by directing the control current through the VCM to angularly accelerate the actuator arm in a first direction and then reversing the control current to angularly decelerate the actuator arm, typically followed by some additional time for the head to settle on the proper target track. Once the head is on the desired track, a track following current is provided to the VCM in order to maintain tracking.
The current supplied to the VCM during a seek operation typically follows a predetermined profile that includes acceleration and deceleration phases. Currently, these profiles are conservatively configured and implemented so that the values used for maximum acceleration and deceleration values leave ample margins between the values and maximum current that is available for use. One reason for such conservative margins is that the maximum current available for use by the VCM varies with the operating conditions (e.g. temperature, supply voltage), and drive parameters (e.g. FET resistance, VCM winding resistance) of the disk drive, both of which affect how much current can be delivered to the VCM. Additionally, during a seek operation, the servo-controller generally has little or no visibility as to the variations in the VCM motor torque parameter (Kt) which also affects the overall seek process. Thus, if during a seek the head is first accelerated to above a maximum allowed acceleration value dictated by the varying operating conditions, drive parameters and the VCM motor torque parameter of the disk drive, then the deceleration phase may not able to stop the head at the target track. In such a situation, the head overshoots the intended target, and a substantial amount of extra time is then required to bring the head back and settle at the target track. Because of such a negative consequence, the acceleration profiles are generally configured conservatively so that the deceleration phase may have ample reserve of current to prevent overshoot during the seek operation. One disadvantage of utilizing conservative current profiles, however, is that some seek time is sacrificed.
The linear gain variations in the VCM motor torque parameter also affect track-follow operations of the actuator. These variations of the VCM motor torque parameter may cause the servo bandwidth to drift away from the pre-set optimum values, resulting in degradation of the performance of drives by increasing the risk of off-track head position, and could even cause the servo system to become unstable as a whole.
Accordingly, what is needed is a servo system that can improve the performance of the actuator by accounting for the variations in the VCM motor torque parameter.